Telecommunications equipment for converting electrical signals to optical signals and for converting optical signals to electrical signals is useful in various applications. For example, electrical-to-optical and optical-to-electrical converters are useful in applications, such as high-definition television (HDTV) applications, where signals arrive in electrical media but must be transmitted or distributed over distances greater than is possible with a continuous electrical conductor. Electrical conductors can carry signals over only relatively limited distances due to signal attenuation. In contrast, fiber optic conductors can carry signals over distances many times greater than electrical conductors. Electrical signals, however, are easier to manipulate for media purposes than optical signals. Therefore, it is common to apply an electrical-to-optical conversion to a signal prior to distribution or long-distance transmission, and then apply an optical-to-electrical conversion near the destination.
Some of the most common optical-electrical converters used in telecommunication and data communication applications are Small Form Factor Pluggable (SFP), Small Form Factor Pluggable Plus (SFP+), and 10 Gigabit Small Form Factor Pluggable (XFP) converters because they are compact, and hot-pluggable. The specific standards for SFP converters are set out in the “Small Form Factor Pluggable (SFP) Transceiver Multi-Source Agreement (MSA)” dated Sep. 14, 2000.
However, most conventional SFP, SPF+, and XFP converters are not suitable for video applications. First, conventional SFP, SFP+ and XFP converters are bidirectional and have both electrical-to-optical and optical-to-electrical conversion circuits. Since video systems commonly use unidirectional signals, half of a conventional SFP, SFP+ or XFP converter is unused or wasted in video systems.
Second, professional digital video signals, such as serial digital interface (SDI) and high-definition serial digital interface (HD-SDI) video signals, contain pathological patterns (patterns with long duration of “0's” or “1's”) which conventional SFP, SFP+ and XFP converters cannot accurately process. One specific pathological pattern consists of 19 bits of one polarity followed by one bit of the opposite polarity. This produces a signal with either a very high or very low duty cycle. Conventional optical transmitter modules have an automatic power control circuit that keeps the optical power output at a set level. These circuits assume incoming data will have a constant average duty cycle of about 50% over a period of time (e.g. 1 μs). Since a pathological pattern may last for up to 63.5 μs, the laser driver output of the optical transmitter would be adjusted too high or two low, depending on the duty cycle of the signal. Accordingly, conventional SFP, SFP+, and XFP convertors cannot typically process professional digital video signal without producing data errors.
Third, conventional SFP, SFP+ and XFP convertors cannot directly receive a single-ended unbalanced signal. Specifically, most conventional SFP, SFP+ and XFP can only receive a differential balanced signal. Therefore most conventional SFP, SFP+ and XFP converters must be connected to an external conversion module to convert the single-ended unbalanced signal received from the cable to a differential balanced signal.
To address these issues, manufacturers have recently developed special video SFP, SFP+, and XFP converters.